The compositions and methods described herein relate to the downhole differentiation of light oil and oil-based filtrates by nuclear magnetic resonance (NMR) methods using oleophilic nanoparticles.
In oil and gas exploration it is desirable to understand the structure and properties of the subterranean formation surrounding a wellbore, in order to determine if the formation contains hydrocarbon resources (oil and/or gas), to estimate the amount and producibility of hydrocarbon contained in the formation, and to evaluate the completion operation parameters for bringing the wellbore into production. A significant tool in this evaluation is the use of wireline logging and/or logging-while-drilling (LWD) or measurement-while-drilling (MWD) for analyzing the near-wellbore formation and near-wellbore fluids. Typically, one or more logging tools are lowered into the wellbore and the tool readings or measurement logs are recorded as the tools traverse the wellbore. These measurement logs are used to infer the properties of the near-wellbore formation and/or the near-wellbore fluids.
Nuclear magnetic resonance (NMR) logging is especially useful for analyzing the composition, viscosity, diffusivity, and location of near-wellbore fluids and the porosity and permeability of the near-wellbore formation, as these relate directly or indirectly to the NMR like spin-density, T1 and the T2 relaxation times, and signal decay rate. NMR logging is based on the observation that when an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field, they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization is characterized by the parameter T1, known as the spin-lattice relaxation time. The T1 parameter characterizes the coupling of nuclear spins to energy-absorbing molecular motions like rotation, vibration, and translation. Another related and frequently used NMR logging parameter is the spin-spin relaxation time T2 (also known as transverse relaxation time), which is an expression of the relaxation due to non-homogeneities in the local magnetic field over the sensing volume of the logging tool. In general, the mechanisms for spin-spin relaxation time T2 include, in addition to those contributing to T1, the exchange of energy between spins.
For accurate NMR logging, the various materials being queried (e.g., the various formation rock and various fluids therein) need to have NMR parameter values. However, the wellbore fluids utilized in wellbore operations (e.g., drilling operations) can have similar NMR parameter values to near-wellbore fluids. As such, fluid differentiation becomes difficult when wellbore fluids infiltrate the subterranean formation, often referred to as filtrates. When the filtrate and the formation fluid have similar NMR parameter values, the properties of the formation fluid can be skewed by the filtrate. Inaccurate NMR parameter values may lead to inaccurate formation fluid properties and consequently the design of an inefficient wellbore completion operation.
Most often, fluid differentiation can be difficult between filtrates from aqueous wellbore fluids and formation water and between filtrates from oil-based wellbore fluids and light oil in the formation. In some instances, wellbore fluids have been doped with NMR contrast agents like chelated gadolinium to assist in fluid differentiation. However, in aqueous-based wellbore fluids, the concentration of chelated gadolinium needed to achieve adequate contrast is sufficiently high that to achieve such a concentration the ratio of chelant to gadolinium increases to a point that the gadolinium is no longer an effective contrast agent.
Oil-based mud is often chosen for wellbore stability in shale formation, in deep or high-temperature wells that dehydrates water-based mud, or drilling through water-soluble formation such as salt. Oil-base filtrates with NMR parameters similar to that of the oil-based formation fluids can magnify any inaccuracy associated with NMR logging methods. Accordingly, enhancing the ability to differentiate oil-based filtrates and oil-based formation fluids such as light oils is important to identify or quantify the reservoir fluids and fluid saturations.